In a photo detector of an optical remote controller, a comparator circuit is used for reproducing an original pulse signal from an analog signal made by demodulating an output from a photo detector element.
An example of a conventional type of comparator circuit equipped with a differential amplifier is shown in FIG. 4.
The differential amplifier 1 is composed of a pair of n-p-n transistors Q.sub.1, Q.sub.2 identical in properties and a first constant current source 2 connected to the emitters of both thereof.
The base of the aforementioned n-p-n transistor Q.sub.1 has applied thereto a reference voltage Va for comparison. It is so arranged that this reference voltage Va is determined by the sum of the voltage across a resistor R.sub.1 and the voltage across a second constant current source 3. The voltage across the aforementioned second constant current source 3 is kept constant and equal to the sum of another reference voltage V.sub.ref and the base-to-emitter voltage V.sub.BE3 of an n-p-n transistor Q.sub.3.
Meanwhile, the voltage across the resistor R.sub.1 varies according to the current I.sub.3 equal in amperage to the collector current I.sub.3 which flows via a current mirror circuit 4.
That is, the reference voltage Va applied to the base of the n-p-n transistor Q.sub.1 is varied according to the operating status of the differential amplifier 1 so that the comparator circuit has a hysteresis property.
An input terminal 5 is connected to the base of the n-p-n transistor Q.sub.2 so that an input voltage VIN is applied thereto. The collector of the n-p-n transistor Q.sub.2 is connected to the comparator's output terminal 6 via a p-n-p transistor Q.sub.6 and an n-p-n transistor Q.sub.7 so that an output voltage V.sub.OUT is obtained.
Hence, by the use of this comparator circuit, it is possible to obtain from the comparator's output terminal 6 an output voltage V.sub.OUT corresponding to the original pulse signal by applying the analog signal of the demodulated output from the photo detector element to the input terminal 5 as the input voltage V.sub.IN.
Such a conventional comparator circuit, however, has a problem in that the input impedance of the input terminal is not sufficiently high.
That is, although the input impedance can be raised if the current I.sub.1 is reduced, it is, however, impossible to sizably reduce the current I.sub.1. For, if the current I.sub.1 is reduced, a collector current I.sub.3 of the n-p-n transistor Q.sub.1 for setting the reference voltage Va is also reduced. Also, if the current I.sub.1 is reduced, a collector current I.sub.4 of the n-p-n transistor Q.sub.2 to drive the n-p-n transistor Q.sub.7 of the output circuit is reduced. Hence, actually, it is difficult to sizably raise the input impedance only by reducing the current amperage I.sub.1 of the first constant current source 2.